This fixes a bug where instantiations of consuming_buffers with
buffer sequence types whose value_type is not const_buffer or
mutable_buffer can cause compilation errors.
The function consumed_buffers is removed.
A new concept Parser is introduced with routines to read from a stream
into the parser. This solves a problem with the old read interface where
messages must be default constructible and move assignable.
Parser fixes:
* Fix detect invalid reason-phrase octets
* Fix write_eof to set the 'complete' state on success
* Fix consider parse complete if eof received on empty body
WebSocket:
* Increase coverage
